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The coexistence of a large variety of molecular species (i.e., aromatic,cycloaliphatic and aliphatic) in several astrophysical environments suggeststhat unidentified IR emission (UIE) occurs from small solid particlescontaining a mix of aromatic and aliphatic structures (e.g., coal, petroleum,etc.), renewing the astronomical interest on this type of materials. A seriesof heavy petroleum fractions namely DAE, RAE, BQ-1, and asphaltenes derivedfrom BQ-1 were used together with anthracite coal and bitumen as modelcompounds in matching the band pattern of the emission features ofproto-planetary nebulae (PPNe). All the model materials were examined in themid-infrared (2.5-16.7 um) and for the first time in the far-infrared (16.7-200um), and the IR bands were compared with the UIE from PPNe. The best match ofthe PPNe band pattern is offered by the BQ-1 heavy aromatic oil fraction and byits asphaltenes fraction. Particularly interesting is the ability of BQ-1 tomatch the band pattern of the aromatic-aliphatic C-H stretching bands ofcertain PPNe, a result which is not achieved neither by the coal model nor bythe other petroleum fractions considered here. This study shows that a newinteresting molecular model of the emission features of PPNe are asphaltenemolecules which are composed by an aromatic core containing 3-4 condensedaromatic rings surrounded by cycloaliphatic (naphtenic) and aliphatic alkylchains. It is instead shown the weakness of the model involving a mixture ofPAHs for modeling the aromatic IR emission bands. The laboratory spectra ofthese complex organic compounds represent a unique data set of high value forthe astronomical community; e.g., they may be compared with the Herschel SpaceObservatory spectra (~51-220 um) of several astrophysical environments such as(proto-) PNe, H II regions, reflection nebulae, star forming galaxies, andyoung stellar objects.

Far- and mid-infrared spectroscopy of complex organic matter of astrochemical interest: coal, heavy petroleum fractions, and asphaltenes